1. Field
The present invention relates to solid catalysts and a fuel cell.
2. Description of the Related Art
Solid polymer type fuel cells, which are characterized by having a high current density, are known as one kind of fuel cells. Of these, direct-methanol fuel cells (DMFCs), which are preferable for size reduction, are being developed enthusiastically.
The reactions occurring on the anode side in a direct-methanol fuel cell (DMFC) are methanol decomposition reactions which proceed in steps on the surface of the metal catalyst. Namely, an anode reaction represented by the following scheme 1 proceeds on the anode side.CH3OH+H2O→6H++6e−+CO2  (scheme 1)
The carbon dioxide generation reaction which is the final step in this anode reaction is one of the reaction-rate-determining steps. The fact that this step is a rate-determining step is generally known to be attributable to “carbon monoxide poisoning” in which the carbon monoxide generated by the proton elimination reaction of the methanol as a fuel is tightly adsorbed onto the platinum surface to thereby reduce the catalytic activity.
In this connection, it is known that the value of active current is greatly improved by replacing the platinum catalyst with a platinum-ruthenium alloy catalyst. This improvement is thought to be attained by the following mechanism. The carbon monoxide generated on the platinum surface is rapidly oxidized by the ruthenium, which has the higher ability to oxidize carbon monoxide than platinum. Because of this, the carbon monoxide poisoning of the platinum surface, which is important for the methanol decomposition reaction, is reduced (see H. A. Gasteiger, N. Markovic, P. N. Ross, E. J. Cairns, J. Phys. Chem., 98, 617 (1994) and S. Wasmus and A. Kuver, J. Electroanal. Chem., 461, 14 (1999)).
Many attempts have been made to improve catalytic activity and heighten the value of active current by reducing the carbon monoxide poisoning of platinum in a higher degree than in the case of using a platinum-ruthenium alloy catalyst.
One of approaches to the poisoning reduction is a “multinary alloy catalyst” obtained by adding one or more other elements to a platinum-ruthenium alloy. However, since there is no clear guideline for design in catalyst composition screening, the method in general use is to experimentally investigate each of many catalyst compositions.
On the other hand, besides composition, it is important to regulate a surface structure so as to have high activity in a desired reaction. However, there are few patent documents concerning a technique for positively controlling the surface structure of an electrode catalyst.
JP-A 2003-157857 (KOKAI) proposes a fuel cell cathode (air electrode) which has a catalyst surface including a large proportion of exposed platinum (001) faces, which have higher activity, to thereby have improved activity. JP-A 2007-220654 (KOKAI) proposes an anode which has a catalyst-alloy surface including exposed (100) faces, (010) faces, and (001) faces to thereby have improved catalytic activity as in the technique described above.
However, in the case of the platinum-ruthenium alloy catalyst containing one or more additional elements, the correlation between the state of distribution of the atoms in the catalyst surface and catalytic activity is not clear.
As described above, the conventional solid catalysts for use in direct-methanol fuel cells (DMFCs) have had a problem concerning carbon monoxide poisoning.